Adaptive equalizer using precursor error signal for convergence control

• US 4,789,994 A
• Filed: 08/12/1987
• Issued: 12/06/1988
• Est. Priority Date: 08/12/1987
• Status: Expired due to Term

• Current Assignee: American Telephone & Telegraph Company (AT&T, Inc.), Bell Telephone Laboratories, Inc. (Nokia Corporation)
• Original Assignee: American Telephone & Telegraph Company (AT&T, Inc.)
• Inventors: Randall, Roderick K., Zucker, William F.
• Primary Examiner(s): Griffin, Robert L.
• Assistant Examiner(s): Chin, Stephen
• Application Number:
  US07/084,356
• Time in Patent Office: 482 Days
• Field of Search: 375/12, 375/14, 375/15, 333/18, 333/28, 364/724, 381/103
• US Class Current: 375/233
• CPC Class Codes: H04L 25/03057 with a recursive structure H04L25/03031 takes precedence

Abstract

Operation of a decision feedback equalizer is enhanced by measuring the intersymbol interference precursor effect on each symbol decision output and utilizing that measurement information to update equalizer, transversal filter, tap multiplier coefficients in a direction to reduce the precursor and postcursor effects. In one embodiment, the updated coefficient information is further utilized to adjust equalizer input signal sampling phase toward a time of maximum data signal eye opening.

Claims

1. In a decision feedback equalizer having a postcursor error effect estimator means in a feedback path thereof

means for estimating predetermined cursor effects, including precursor error effect, in an output of said equalizer,

means, responsive to an output of said predetermined cursor effect estimating means, for developing an error signal which is indicative of the size of at least said precursor error effect, and

means, responsive to said error signal, for modifying operation of both of said predetermined cursor and postcursor estimating means in a direction to reduce respective error effects in said equalizer output.

2. The decision feedback equalizer in accordance with claim 1 in which there are provided

means for recurrently sampling data signals at an input to said equalizer,

means, responsive to said modifying means, for measuring eye height of a data signal in an output of said equalizer, and

means, responsive to said measuring means, for adjusting sampling time of data signals in said sampling means to maximize the steady state value of said eye height.

3. The decision feedback equalizer in accordance with any of claims 1 and 2 in which each of said estimating means comprises

a transversal filter having a tapped delay line, a plurality of tapping point circuits each connected to a different tapping point along said delay line and each having tap signal multiplier means for multiplying the respective tapping point signals by respective impulse-response-representative coefficient estimate signals, and means for summing multiplier means output signals from all tapping point circuits of such estimating means to produce an estimating means output signal,

a plurality of tap logic circuits to provide respective coefficient estimate signals to each of said multiplier means,

means for applying said error signal to a first input of each of said tap logic circuits,

means for appyling a predetermined different one of said tapping point signals to a second input of each of said tap logic circuits, and

means, in each of said tap logic circuits and responsive to signals at said first and second inputs thereof, for adapting said respective coefficient estimate signal of said each tap logic circuit to reduce said error signal.

4. In a decision feedback equalizer for use with respect to digital data input signals subject to intersymbol interference comprising both precursor and postcursor error effects, said equalizer including a feedback path transversal filter for postcursor error estimation, said filter having a set of controllably variable, impulse-response-based, operation determinative coefficients, and said equalizer further includes means for adapting said filter coefficients to variable transmission conditions, said adapting means comprising

means for measuring predetermined cursor effects, including at least said precursor error effect, on each baud time output of said equalizer, and

means, responsive to an output of said measuring means, for modifying said filter coefficients in a direction to reduce said postcursor error effect in said equalizer output.

5. The decision feedback equalizer in accordance with claim 4 in which said filter comprises

a tapped delay line for receiving signals from said equalizer output,

a plurality of tap circuits, each corresponding to a different one of said coefficients connected at respective predetermined tapping points along said delay line and each having means for multiplying its coefficient by its tapping point signal to produce a tap output signal,

means for summing all of said tap output signals from said tap circuits to produce a filter output signal that is an estimate of said postcursor error effect, and

said tap circuits each including,

means for multiplying said output of said measuring means by a tapping point signal one data signal symbol time prior to the symbol time corresponding to said each tap circuit, and

means responsive to an output of said modifying means, for adapting said coefficient for such tap circuit in a direction to reduce said postcursor error effect.

6. The decision feedback equalizer in accordance with claim 5 in which

said means for multiplying said output of said measuring means comprises means for EXCLUSIVE ORing said error signal and said one-symbol-time-prior tapping point signal, and

said adapting means comprises means for storing a coefficient estimate to be available at an output of said storing means, and means for incrementing or decrementing a predetermined bit order position of the last-mentioned coefficient estimate depending upon the binary signal state of the output of said EXCLUSIVE ORing means whereby such coefficient estimate is adapted according to said error signal.

7. The decision feedback equalizer in accordance with claim 5 in which said measuring means comprises

a precursor estimator transversal filter connected between said equalizer output and said coefficient modifying means, said precursor filter having a set of controllably variable, impulse-response-based, operation determinative, coefficient estimates, and said filter comprising

a tapped delay line for receiving signals from said equalizer output, a plurality of tap circuits, each corresponding to a different one of said precursor filter coefficient estimates, connected at respective predetermined tapping points along said precursor filter delay line and each having means for summing all of said precursor filter tap output signals from said tap circuits of such precursor filter to produce a filter output signal that includes an estimate of said precursor error effect, and said precursor filter tap circuits each including,

means for multiplying said output of said measuring means by the precursor filter tapping point signal for the same symbol time as the symbol time corresponding to said each precursor filter tap circuit, and

means, responsive to an output of said modifying means, for increasing or decreasing said coefficient for such tap circuit in a direction to reduce said precursor error effect.

8. The decision feedback equalizer in accordance with claim 7 in which there are provided

means for recurrently sampling said digital data input signals in predetermined phases thereof,

said precursor filter has at least two of said tap circuits, a main cursor symbol time tap circuit and a precursor symbol time tap circuit, providing a main cursor coefficient estimate and a precursor coefficient estimate, respectively,

means for comparing coefficient estimate differences between said main and precursor coefficient estimates for symbol times separated by a plurality of symbol times sufficient to allow said equalizer to stabilize its phase,

means, responsive to an output of said comparing means, for developing a binary signal representative of the direction of change in comparing means output polarity, and

means, responsive to an output of said developing means, for changing the phase of operation of said sampling means in a direction tending to maximize the amplitude of said coefficient estimate differences of a predetermined polarity.

9. The decision feedback equalizer in accordance with claim 4 in which

said equalizer includes a forward signal transmission path having a signal level decision circuit for producing an equalizer output signal,

said measuring means comprises precursor estimator means coupled to an output of said decision circuit for developing a signal having a level which is indicative of the extent of said precursor effect,

said coefficient modifying means comprises

means coupled to an input of said decision circuit for developing a representing of signals at said input but delayed in time by an amount corresponding to signal delay during transmission through said decision circuit and said measuring means,

means, responsive to outputs of said measuring means and said developing means, for producing an error signal having a binary state indicative of the difference between such outputs, and

coefficient update logic means, responsive to said error signal and to an output of said decision circuit, for updating coefficients of said precursor equalizer in a direction to reduce said error signal.

10. The decision feedback equalizer in accordance with claim 9 in which

means are provided for coupling further outputs of said coefficient update logic means to provide to said feedback path transversal filter coefficients updated in a direction to reduce said postcursor effects.

11. The decision feedback equalizer in accordance with claim 4 in which said input signals are binary coded signals in which time superimposed traces of successive bauds define an enclosed geometric figure having an opening eye of a height which is a function of said intersymbol interference and in which equalizer there are provided

means for recurrently sampling said input signals in an adjustable phase time of each baud period to cause operation of said equalizer with respect to discrete successive samples thereof, and

means, responsive to coefficients from said adapting means, for adjusting the phase of operation of said sampling means toward a time of maximum, presample, digital signal eye.